Hydroxyapatite nanoparticles (HANPs) were suspended in injectable, biodegradable, phosphate-containing, dual-gelling macromer solutions that were used to encapsulate mesenchymal stem cells (MSCs) within stable hydrogel composites when elevated to physiologic temperature. The suspension of HANPs at 0.75%w/v within the hydrogels was found to have no significant effect on the swelling ratio or the compressive modulus. The MSCs were shown to survive the encapsulation process, and live cells were detected within the hydrogel composites for up to 28 days. The activity of osteogenic marker alkaline phosphatase increased with time in cell-laden hydrogel composites over the 28 days in osteogenic medium, suggesting that the joint combination of encapsulated MSCs within a HANP- and phosphate-containing hydrogel can enhance enzyme activity to assist in hydrogel mineralization. This is observed in the calcium biochemical assays, where the incorporation of HANPs significantly improved both cell-laden and acellular hydrogel composite mineralization over time. Hydrogel nanocomposites that form in situ while facilitating cell delivery and mineralization are promising materials for craniofacial bone tissue engineering.